1. Field of the Invention
The present invention relates to a printing apparatus which includes an image forming means utilizing, for example, an electrophotographic process, a control method for the printing apparatus, and a computer-readable storage medium storing a control program for the printing apparatus.
2. Description of the Related Art
Page printers, primarily laser beam printers, have developed rapidly in recent years. More recently, a variety of page printers capable of outputting full-color images have also been proposed and commercialized.
In a color laser beam printer, a full-color image is formed by superimposing four images which are formed with the known electrophotographic process using toners of four colors. Various systems have been proposed for carrying out the electrophotographic process in a full-color fashion.
For example, an intermediate transfer system is known wherein a full-color image is formed by developing a latent image on a photoconductive drum into a toner image of each color, transferring each toner image to an intermediate transfer member successively to superimpose four-color toner images on the intermediate transfer member, and then transferring the four-color toner images to a sheet of paper at a time, followed by fusing the transferred images.
With such an intermediate transfer system, as shown in FIG. 11, toner images 1001-1004 of four colors, i.e., C (cyan), M (magenta), Y (yellow) and K (black, also denoted by Bk hereinafter), are successively formed on a photoconductive drum and transferred to an intermediate transfer member 1100 in superimposed relation. Then, the four-color toner images are transferred from the intermediate transfer member 1100 to a sheet of paper 1200. Note that while the photoconductive drum and the intermediate transfer member each having a cylindrical shape are usually employed, image forming surfaces of those components are shown in a developed state in FIG. 11 and FIG. 12 (described later).
The intermediate transfer system is advantageous in that since the positional relationship between the photoconductive drum and the intermediate transfer member is physically fixed, it is easier to avoid displacements of the four-color images than a system of transferring a toner image to a sheet of paper for each color without using the intermediate transfer member.
However, the image forming method based on the intermediate transfer system has such a problem that an intermediate transfer member having dimensions which correspond to a maximum size of sheet of paper available is required, and a throughput of printing is reduced when using sheets of paper smaller than the maximum size.
More specifically, as shown in FIG. 12, assuming that the maximum sheet size supportable by a printer is x.times.y, an intermediate transfer member also must have the size of x.times.y. The left side in FIG. 12 represents the case in which an image is transferred from the intermediate transfer member 1100 to the sheet of paper 1200 having the maximum size. When the sheet of paper 1200 used has a size half the maximum one, i.e., (x.times.y)/2, only a half of the intermediate transfer member 1100 is employed to print one page.
In other words, the problem experienced in the intermediate transfer system is that when an image is formed in units of pages on each sheet of paper having a size smaller than the maximum one, the maximum number of sheets printable per minute cannot be increased over that obtained when printing images on sheets of paper having the maximum size no matter which size of paper is used in printing.